Comment by unanswered | Hacker News Reader

Comment by unanswered | Hacker News Reader

Something I don't understand: when they say "X is 1 minute from happening", does that mean it's really 1 minute from happening or does that mean "in 1 minute we'll receive the signal that X has happened"?

Maybe my question makes more sense in the case of "X is happening right now", because then I should either understand "we infer that X should have happened right about now" or "we have confirmed via signal that X has happened", and that's a big big difference.

I know in some cases they explicitly say the latter, so I guess my real real question is, do they just keep the communication delay implied in all countdowns & references in discussion, to avoid confusion?

(ETA: No need to let me know about simultaneity problems in relativity — earth and mars are, relative to c and to macroscopic time scales, essentially not moving relative to each other AFAIK, so that simultaneity is essentially well-defined. My question was about a much more boring classical-universe problem.)

Yes, those are all Earth Receive Time, that is, when they were saying that eg. entry interface is two minutes away, in reality the rover was already sitting on the surface and we were just waiting for the radio signal to get here.

> Earth Receive Time

Ah great, that's a great phrase to make everything clear and provide a kind of "frame of reference" to think & communicate in. Always need these abstractions.

The local Czech stream (20k viewers!) I watched went over the events in real time and then commented how events were happening as signals were received on Earth - with the final confirmation of successful landing coming first via Twitter, no less! :)

Still a very nice yet nerve wrecking idea to do it like this - you know the lander is on Mars now. But is it safely on the ground or is there a third Shapirelli crater now ? You don't know! A huge relief in the end. :)

But aren’t those two events essentially simultaneous in the relativistic sense? That is by some definitions of “simultaneous”?

In one sense, light experiences no time during travel, so anytime you are hit by radiation (like from a star) there is frame of reference in which the event was instantaneous.

On the other hand, if you were on Earth and I was in between Earth and Mars, I would receive the data more quickly than you, and I could even watch it whiz by me on its way to you. The thing about relativity is that it’s... relative!

Waiting for the physicist in the room to point out: there is no such thing as simultaneity!

:)

Came here to suggest The Order of Time by C Rovelli, which explains this in such a captivating way.

I think this video explains the issue quite well in only two minutes:

https://www.youtube.com/watch?v=wteiuxyqtoM

Another one here, it is in Catalan but with subtitles in english. Minute 18 is where it is explained although I think is worth it to watch all of it

https://www.ccma.cat/tv3/alacarta/quequicom/tempus-fugit-sub...

But that only happens with sufficiently high relative velocities. Earth and Mars are effectively in the same reference frame.

S/he already did. We just don't know yet.

She both did and did not until we observe :p.

And even that is not correct. Events happening propagate with speed of light - the event horizon. We can predict we will receive information of something happening, bit that's just prediction about future events, regardless of the location.

You can just say "they".

(Not trying to be judgy. People just seem to forget about the gender neutral use of "they".)

There are like 500 other ways you could have said what you just said as well. Doesn’t mean the way you said it is invalid.

Why is it acceptable to use the wrong pronoun, such as "they", for someone who chooses the pronoun "she" or "he"?

> used to refer to a person of unspecified gender.

How is this offensive to anyone??

For the same reason referring to anyone with the wrong pronouns is, I would assume. Isn't that offensive? I didn't realize this was up for debate in 2021.

But you're not referring to someone with the wrong pronoun, but using a gender neutral one.

I'm sorry, I honestly don't understand what distinction you're trying to point out. If someone's preferred pronoun is "she", for example, and I refer to her as a "they", then that's the wrong pronoun, isn't it? That's literally the definition of "wrong", at least the definition I understand. The right pronoun is "she" and other pronouns such as "he" and "they" and "it" are, by exclusion, wrong.

But it seems like you're saying there's some kind of complex relation where sometimes people don't get to choose their own pronouns, but other people get to choose which one out of many to use based on convenience. Maybe it would help my understand if you could provide a chart relating the pronouns someone chooses with the pronouns other people are then allowed to use?

> Maybe it would help my understand if you could provide a chart relating the pronouns someone chooses with the pronouns other people are then allowed to use?

I think this is everything wrong with the world currently. Provide you with a chart, so I can justify using gender neutral pronoun?

https://apastyle.apa.org/blog/singular-they

This is barely more than a year old, is it already outdated? I am an asshole for daring to use “they”?

> If a person uses “she” or “he,” do not use “they” instead. Likewise, if a person uses “they,” do not switch to “he” or “she.” Use the pronouns the person uses.

This matches my own understanding; I have no idea why you are referring to this article as if it supports your bizarre crusade to misgender people.

You are aware that we are talking about the mars rover, right?

The comment that you replied to:

> You can just say "they".

Was a reply to this:

> S/he already did. We just don't know yet.

Which was talking about the mars rover.

They are not talking about a specific person, so how is "they" wrong?

I'm not a native speaker but the problem I have with it is that it has the wrong numerus. Although I have done so in papers recently, because it seems to be a trend, sentences like "If a person chose option B, they were categorized as a cautious assessor" seem ungrammatical to me. (In this case it's easy to reformulate the sentence in plural and simpler, but that's not always the case and I hope you get the point.)

If the person is hypothetical or not fleshed-out and without a gender, like your example, they is fine and I don't even notice. But if the person is known, the use of they catches me off guard every time. I recently read a book by Brandon Sanderson that had aliens on another planet with a different gender system, so Sanderson just used they to refer to those aliens even though the characters were obviously either feminine or masculine. It completely broke the illusion of the story and was a complete turn-off. I always notice in such cases, but for some reason some people say it's totally standard English.

Ann Leckie has written a great sci-fi series, Ancillary Justice/Ancillary Sword/Ancillary Mercy, where the main character assumes everyone is a she and keeps referring to them as that all the way through (though it becomes fairly obvious to the reader which are hes and shes).

I found it actually fairly annoying at first, but quickly got over it because it is related to the plot. Same for Sanderson, I've read all his books and am not even sure now which one you're talking about!

Sometimes we need a nudge to notice the unconscious bias around us, and it's often uncomfortable.

It was Starsight (Skyward series, book 2). The diones are apparently genderless or have unknown genders, and reproduce by merging into a single being for a while so they can "try out" the adult version of the kid they're going to have. If they don't like the kid they can reject that version and try again (eugenics), otherwise the actual kid is born from an egg or something.

The whole genderless thing was completely irrelevant to the plot—the diones could have been male or female and little else would have changed. It's not that it made me uncomfortable, but it was an unnecessary distraction.

Anyway, the book was a 2/5 because it threw almost everything from book 1 away, had way too many characters and places, ended on a serious cliffhanger, and the fictional science was extremely far-fetched. Meanwhile, book 1 was fantastic (a full 1/1 in my opinion) was focused on a small cast of characters on a small set, had great character development, and resolved nicely.

> > the problem I have with it is that it has the wrong numerus.

Not really: https://www.merriam-webster.com/words-at-play/singular-nonbi...

Simultaneity is at least as well defined as clock time. Both clock time and the velocity of your reference frame can have arbitrary constants added to them to yield equally valid coordinate systems. So "it's not really simultaneous" is analogous to "it's not really 6:30 PM."

Well, it's 6:30 PM somewhere.

lifts glass

From The Hitchhiker's Guide to the Galaxy: "Time is an illusion. Lunchtime doubly so."

> simultaneity

not in the true accurate to the picosecond sense of the word, no, but the exact word simultaneity is used when discussing number and density of satellites about a given latitude/longitude in the starlink beta program. Since they're LEO and orbiting at only 550 km, the satellites above a given spot on the ground vary greatly in the not-yet-complete sparse network.

Usually related to discussions of whether a beta test customer terminal will briefly hiccup and lose connection to its default gateway, or if somebody is at a sufficiently high latitude that they can have full coverage for all 86400 seconds in a day.

https://satellitemap.space/ has a good animated visualization of this.

Depends what type of physicist you ask.

According to the energy-time uncertainty principle we don't even know when exactly the RF waves that transmitted information hit the receiver on Earth either.

Oh but there is. Just not in the same frame of reference.

In the absence of a physicist, I suppose that a software engineer in the room might do. After all, the counterintuitive consequences of relativity have their counterparts in counterintuitive effects in distributed systems and concurrent programming. In both cases, the core issue that misleads our intuition is the lack of a shared global clock that would impose a total ordering [1] on events/reads/writes/etc. Instead, events in both situations are only ordered partially [2]. In relativity the ordering is determined by the speed of light, in distributed systems the ordering is determined by what messages have been exchanged by two nodes and in concurrent programming reads and writes are ordered by synchronization actions such as lock acquisition and release, memory barriers etc (c.f. the happens-before relationship in JMM [3] and other memory models).

See for example [4] and [5].

[1]: https://en.wikipedia.org/wiki/Total_order

[2]: https://en.wikipedia.org/wiki/Partially_ordered_set

[3]: https://en.wikipedia.org/wiki/Java_memory_model

[4]: https://www.youtube.com/watch?v=UYZIHP120go

[5]: https://www.microsoft.com/en-us/research/publication/time-cl...

It's the latter. The Earth-Mars latency at this time is something like 11 minutes, and the landing itself takes about 7 minutes, so when we on Earth first saw the craft entering atmosphere on Mars, by that time all the landing was already over, one way or another.

NASA has a good breakdown of their expected miletones at 'earth receive time' - https://www.nasa.gov/feature/jpl/nasa-s-next-mars-rover-is-r...

Apparently the latency time is currently 11 mins 22 seconds -- which is somewhere near the average. It goes from under 4 mins to over 22mins depending on distance.

It looked like they were quoting time as it would appear for an earth-local observer (i.e. a million light-year away supernova that showed up five minutes ago happened "five minutes ago" not 1 million years and five minutes ago.

With your personal light cone, it's fine to equate "now" with what you see in the moment. It just has to be clear what you mean for situations where communication might be ambiguous. If you have a person on mars, be sure to be precise what you mean when you tell them to do something in five minutes, when they receive the message they won't know if you mean five minutes after they receive the message or anywhere between 17 minutes before and 2 minutes after they receive the message.

When you get into relativistic speeds (and especially very short time intervals), nobody can even agree on when something "actually" happened, different observers have different opinions about what happens when even after you account for light travel time.

Relative time (in five minutes) without relativistic speeds is actually uniform. The is no observable difference to either participant.

And there is no concept of now in a significantly distant location. Related video: https://youtu.be/pTn6Ewhb27k

Doesn’t relativity tell us it doesn’t matter?

There's a lag time in communication due to distance. I don't see what that has to do with relativity.

PBS Space Time recently explained what the present time means within general relativity[1]. As I understand it... it matters in this context.

[1]: https://www.youtube.com/watch?v=EagNUvNfsUI

True fun begins when you consider General Relativity (which takes into account gravity and acceleration). From what I heard there is no definition of simultaneity and you can define it in different ways.

It depends on what coordinate system you're using. Simultaneity is ill defined in relativity. There's only future, past and "spacelike-separated" (neither past nor future). When they say, "X is 1 minute from happening," it's actually neither in the past nor the future. It's currently spacelike-separated, but in 1 minute, it will be in our past.

Yes, yes, you have shown you know what relativity is. But the relative velocity of earth and mars — which I can't convince Wolfram Alpha to tell me, but it's got to be on the order of their orbital velocity so let's say 5x10^4 mph — is a tiny tiny fraction of c so their inertial reference frames are essentially identical. So sitting in our reference frame, we can make inferences about what's happening "now" on mars,such that these inferences are consistent (to within that tiny fraction of c) with all of our current and future observations in this reference frame; i.e., consistent with a classical(+ finite speed of light) model of the universe. Which is why I left this out of my question and only asked about the consequences of a finite speed of light.

Put another way, simultaneity is perfectly well defined in a single inertial reference frame, and for purposes of my question, earth and mars can be considered to be relatively motionless.

Now is the part where someone brings up even with the low relative velocity, you haven’t accounted for the simultaneity issues we’d have if the earth collided with a black hole while the rover landing was happening.

Or the fact that mars is not as deep in the gravity well of the Sun! I wonder if they have to account for that one when programming, like, antenna aiming or something.

IIRC gps suffers about 1hz of blueshift due to descending into earths gravity. I think the velocity (Doppler) shift of the spacecraft is a way bigger factor than the gravitational shifting.

No, I don’t think you’re quiet understanding what they are saying. They aren’t talking about the different speeds of earth or Mars.

Simultaneity is not “perfectly well defined in a single intertidal reference frame”. That is just a convention.

If the RTT of earth to Mars is 20 minutes, then we can say that it takes us 20 minutes for our message to reach the rover, and the rover’s message arrives instantly, and that’s a consistent definition of simultaneity.

https://www.pitt.edu/~jdnorton/teaching/HPS_0410/chapters/si...

Except that an observer located in the (hypothetical/approximate) common reference frame, but situated halfway to Mars, will report observations inconsistent with this definition (NB of course we receive their report at a time consistent with the definition, but the contents of the report are not consistent). So yes, you can play games with your definition of simultaneity, but you will win stupid prizes like observers in the same reference frame no longer agreeing about simultaneity when such a result is worse than what relativity requires.

Your link points this out. You can play these games; I don't dispute it. But it's a separate matter entirely from anything to do with relativity, as your link points out, which is itself separate from the classical problem I originally posed. So we are now two steps removed from anything relevant to the Mars rover; I guess we get a sense of pride and accomplishment?

It doesn't depend on the relative velocity of Earth and Mars. It depends on your coordinate system. You can use Schwarzschild coordinates centered on the Sun and use the time coordinate to define simultaneity, but that's an arbitrary choice.

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